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    Subject[RFC][PATCH 0/2] cgroup: Fix some races against css_set task links
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    Hi,

    This is trying to fix some races in the way to link all the tasks to
    the css_set links. I hope some people can have a look at this, especially
    as I'm definetly not an SMP ordering expert.

    To give you the big picture, as long as nobody never calls
    cgroup_iter_start(), we don't link the tasks to their css_set (this 'link'
    is a simple list_add()).

    But once somebody calls cgroup_iter_start(), we call
    cgroup_enable_task_cg_lists() that grossly does this:

    cgroup_enable_task_cg_lists() {
    use_task_set_css_links = 1;
    do_each_thread(g, p) {
    link p to css_set
    } while_each_thread(g, p);
    }

    But this links only existing tasks, we also need to link all the tasks
    that will be created later, this is what does cgroup_post_fork():

    cgroup_post_fork() {
    if (use_task_set_css_links)
    link p to css_set
    }

    So we have some races here:

    - cgroup_enable_task_cg_lists() iterates over the tasklist
    without protection. The comments are advertizing we are using RCU
    but we don't. And in fact RCU doesn't yet protect against
    while_each_thread().

    - Moreover with RCU there is a risk that we iterate the tasklist but
    we don't immediately see all the last updates that happened. For
    example if a task forks and passes cgroup_post_fork() while
    use_task_set_css_links = 0 then another CPU calling
    cgroup_enable_task_cg_list() can miss the new child while walking the
    tasklist with RCU as it doesn't appear immediately.

    - There is no ordering constraint on use_task_set_css_links read/write
    against the tasklist traversal and modification. cgroup_post_fork()
    may deal with a stale value.

    The second patch of the series is a proposal to fix the three above
    points. Tell me what you think.

    Thanks.

    Frederic Weisbecker (2):
    cgroup: Remove wrong comment on cgroup_enable_task_cg_list()
    cgroup: Walk task list under tasklist_lock in
    cgroup_enable_task_cg_list

    kernel/cgroup.c | 23 ++++++++++++++++++++---
    1 files changed, 20 insertions(+), 3 deletions(-)

    --
    1.7.5.4



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